1. Field of the Invention
The present invention relates to disk drives for computer systems. More particularly, the present invention relates to a disk drive employing VCM demand current to calibrate VCM IR voltage for velocity control of an actuator arm.
2. Description of the Prior Art
Disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and embedded servo sectors. The embedded servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo control system to control the velocity of the actuator arm as it seeks from track to track.
There are times when the servo control system does not have access to the embedded servo sectors yet it is still desirable to control the velocity of the actuator arm. For example, in disk drives wherein the head is parked on a landing-zone of the disk, it is desirable to control the velocity of the actuator arm to unlatch the head during spin-up. In disk drives employing ramp loading/unloading, it is desirable to control the velocity of the actuator arm so that the head is not damaged as it travels off the ramp onto the disk as well as off the disk onto the ramp. Another example is if the servo control system loses servo sector synchronization it is desirable to control the velocity of the actuator arm to facilitate re-synchronizing to the servo sectors.
Prior art techniques for controlling the velocity of the actuator arm when servo sector information is unavailable include using a voltage loop with the detected back EMF generated by the VCM as the feedback. The VCM is essentially an RLC circuit where R is resistance, L inductance, and C the inertia of the motor and load. The voltage contribution of C to the measured back EMF is proportional to the velocity of the VCM. Since the resistance R is in series with C, it is desirable to cancel R""s contribution to the back EMF leaving only LC. Once the resistance R is canceled, at low frequencies Ldi/dt is small leaving the voltage contribution of C as the dominant factor in the measured back EMF.
Prior art techniques for performing VCM resistance compensation include calibrating and subtracting from the measured back EMF the voltage contribution of R (i.e., the IR voltage where I is the current in the VCM). The VCM resistance R is measured by applying a fixed current to the VCM in order to press the actuator arm against a fixed object (e.g., the crash-stop for stopping the head at the inner diameter (ID) or the outer diameter (OD)). With the actuator arm pressed against the fixed object, the velocity is zero and Ldi/dt is zero, leaving the VCM resistance R as the only contribution to the measured back EMF.
If the IR voltage is completely canceled from the measured back EMF, it would result in an under-damped or unstable system. Thus, it is desirable to decrease the measured VCM resistance R by a small offset to leave a relatively small amount of IR voltage in the measured back EMF. In the past, the appropriate offset has been determined analytically by analyzing the deterministic characteristics of the VCM circuitry for a family of disk drives. This characterization can be difficult, and the nominal offset selected must account for the worst case disk drives on both ends of the spectrum. An offset which is too small can lead to instability in some disk drives, and an offset which is too large can lead to poor and possibly faulty performance in other disk drives. This problem is alleviated by using higher performance VCMs (e.g., VCMs with a higher Km and tighter tolerances); however, this increases the cost of the disk drive.
There is, therefore, a need to reduce the cost of a disk drive by employing a less expensive VCM wherein the VCM IR voltage can be accurately calibrated to enable velocity control of an actuator arm when embedded servo information is unavailable.
The present invention may be regarded as a disk drive comprising a disk, a head, an actuator arm for actuating the head radially over the disk, a fixed object, and a voice coil motor (VCM) for rotating the actuator arm about a pivot, the VCM comprising a coil comprising a VCM resistance R. A back EMF voltage detector measures a back EMF voltage across the coil, and a current detector detects a current I flowing through the coil. An IR voltage detector, responsive to the current I detected by the current detector, detects an IR voltage proportional to the current I times the VCM resistance R. A voltage compensator substantially cancels the IR voltage from the measured back EMF voltage to generate a compensated back EMF voltage. A control voltage generator, responsive the compensated back EMF voltage, generates a control voltage applied to the coil to generate the current I flowing through the coil. A voltage calibrator, responsive to the current I detected by the current detector, calibrates the IR voltage detector by generating a calibration command input applied to the control voltage generator to move the actuator arm until it presses against the fixed object. The IR voltage detector is programmed to detect an initial IR voltage, and then adjusted to incrementally change the detected IR voltage until the current I detected by the current detector reaches a predetermined level.
In one embodiment, the initial IR voltage is a low value and the detected IR voltage is incrementally increased. In another embodiment, the initial IR voltage is a high value and the detected IR voltage is incrementally decreased. In yet another embodiment, the IR voltage detector comprises an amplifier. In still another embodiment, the current detector comprises a sense resistor in series with the coil. The present invention may also be regarded as a method of controlling velocity of an actuator arm in a disk drive. The disk drive comprises a disk, a head, the actuator arm, a fixed object, and a voice coil motor (VCM) for rotating the actuator arm about a pivot. The VCM comprises a coil comprising a VCM resistance R. A control voltage is generated from a calibration command input and a detected IR voltage. The control voltage is applied to the coil to generate a current I flowing through the coil to move the actuator arm until it presses against the fixed object. A current I flowing through the coil and the IR voltage proportional to the current I times the VCM resistance R are detected. The detected IR voltage is incrementally changed until the detected current I reaches a predetermined level.